git-gui--askyesno: fix funny text wrapping

The text wrapping seems to be aligned to the right side of the Yes
button, leaving an awful lot of empty space.

Let's try to counter this by using pixel units.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

git-gui/git-gui--askyesno

@@ -20,8 +20,8 @@ if {$argc < 1} {
 }
 
 ${NS}::frame .t
-${NS}::label .t.m -text $prompt -justify center -width 40
-.t.m configure -wraplength 400
+${NS}::label .t.m -text $prompt -justify center -width 400px
+.t.m configure -wraplength 400px
 pack .t.m -side top -fill x -padx 20 -pady 20 -expand 1
 pack .t -side top -fill x -ipadx 20 -ipady 20 -expand 1